Effect Of Aluminum Isomorphic Substitution On Sorption Of Diclofenac To Iron Oxides

Iron oxides in the environment usually contain many metal impurities,the most common of which is Al.The incorporation of Al could change the physicochemical properties of minerals,even hinder the crystallization and nucleation of minerals,thus affecting the sorption behavior of polar functional group pollutants（such as diclofenac（DCF））.In this study,goethite（G-0）,Al-goethite（G-5）,mixed phases of Al-goethite and Al-hematite（G&H-10）,and Al-hematite（H-15）were successfully prepared by coprecipitation method.A series of methods were used to study the effects of Al substitution on the physicochemical properties of iron minerals.The sorption mechanisms of DCF to four minerals were investigated by batch experiments and infrared characterization results.The main findings are as follows:（1）The substitution of Al could lead to the transformation of mineral phase,which was shown in the inhibition of nucleation growth of goethite and direct transformation of ferrite into hematite.The lattice parameters of minerals shifted from the Vegard line under the collective influence of Al substitution and the incorporation of excessive hydroxyl groups.The replacement of Fe³⁺by Al³⁺led to the change of specific surface area and p H_pzc of iron minerals.The change of specific surface area was related to the change of crystal morphology.The difference of p H_pzc might be caused by the variation of crystal plane proportion and the change of mineral phase.Al substitution in goethite structure resulted in the formation of structural defects and the introduction of non-stoichiometric hydroxyl groups,consequently,the content of hydroxyl groups in G-5 was higher than that of G-0.In addition,the presence of Al reduced the solubility of the precursor,which led to the direct formation of hematite from the internal rearrangement and dehydration of the precursor.Therefore,compared with G-0 and G-5,the hydroxyl content of H-15 was lower.（2）Sorption properties of four minerals for DCF were studied.The results showed that G-5had stronger sorption affinity than G-0 due to the increase of p H_pzc and the incorporation of non-stoichiometric hydroxyl groups;the sorption capacity of DCF on H-15 was lower than that of G-5 due to the decrease of sorption sites after mineral phase transformation.DCF interacted with the hydroxyl groups of iron minerals through its carboxyl groups.According to the results of batch experiments and FTIR,H-bonding was the main sorption mechanism for all minerals.DCF was connected with G-0/G-5 by solvent-surface hydration-separated ion pair with a weak H-bonding interaction,meanwhile,DCF was connected with G&H-10/H-15 by surface hydration-shared ion pair with a strong H-bonding interaction.（3）The sorption of DCF on four minerals was p H dependence.When p H was lower than 6.15,the change of p H had negligible effect on DCF sorption.When p H was in the range from 6.15 to p H_pzc,due to weaker promotion of electrostatic force on the formation of H-bond and the instability of H-bond,sorption of DCF decreased sharply with the increasing p H.When p H was higher than p H_pzc,sorption was inhibited due to the repulsion of negative charges on both DCF and mineral surface.